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PhysicsMediumClass 11
Work-Energy Theorem — Net work = ΔKE
Work, Energy & Power
7
JEE Qs
8%
Hard
60
min
Always identify all forces acting on the system and meticulously calculate the work done by each to find the correct net work before equating it to the change in kinetic energy.
🧮 Key Formulas
W_net = ΔKE
ΔKE = KE_final - KE_initial = (1/2)mv_f^2 - (1/2)mv_i^2
W_constant_force = F . S = FS cos θ
W_variable_force = ∫ F . ds
KE = (1/2)mv^2
✅ Key Points for JEE
- 1The Work-Energy Theorem states that the net work done by ALL forces (conservative and non-conservative) acting on an object equals the change in its kinetic energy.
- 2W_net is the scalar sum of work done by each individual force acting on the object over the displacement, not just a component of the net force.
- 3The Work-Energy Theorem is a powerful alternative to Newton's laws of motion, especially when forces are variable, paths are complex, or time is not explicitly required.
- 4This theorem is always valid in an inertial frame of reference, regardless of whether the forces are conservative or non-conservative.
⚠️ Common Mistakes
- ✕Failing to include work done by all forces (e.g., friction, applied forces, normal forces if they do work) in the calculation of W_net.
- ✕Confusing the Work-Energy Theorem with the principle of Conservation of Mechanical Energy; the former is always true, while the latter is a special case.
- ✕Incorrectly calculating the work done by a specific force due to wrong sign convention or misunderstanding the angle between force and displacement vectors.
NCERT Chapters
- Class 11 Physics Ch 6: Work, Energy and Power